1. Field of the Invention
This invention relates to methods of mapping magnetic fields with a fiber optic interferometer wherein an optical fiber is clad with a magnetostrictive material. Field mapping is facilitated by placement of the optical fiber substantially transverse to the magnetic field lines.
2. Description of the Prior Art
A form of a Mach-Zehnder interferometer using single-mode optical fibers for the two arms of the device as a means of measuring pressure and temperature is known. Such a device is analyzed in "Fiber-Optic Sensing of Pressure and Temperature" G. B. Hocker, Applied Optics Vol. 18 No. 9, 1 May 1979, pps. 1445-1448. Similar to the device described by Hocker is a laser fiber-optic interferometric strain gage described in U.S. Pat. No. 4,191,470, issued Mar. 4, 1980 to C. D. Butter.
Furthermore, A. Yariv and H. V. Winsor have proposed the detection of weak magnetic fields by magnetostrictive straining of optical fibers in "Proposal For Detection Of Magnetic Fields Through Magnetostrictive Perturbation Of Optical Fibers" Optic Letters Vol. 5, No. 3 March 1980, pps. 87-89. Therein, it is stated that the output of a magnetostrictive clad optical fiber and a reference fiber (which is insensitive to an external magnetic field) can be mixed in a standard interferometric system, with the phase delay in the magnetostrictive fiber due to the change in the length and index of refraction of the clad fiber, producing an interference pattern between the outputs of the fibers.
The computations of Yariv and Winsor are extended in "Magnetic Field Sensitivity Of An Optical Fiber With Magnetostrictive Jacket", J. Jarzynski, J. H. Cole, J. A. Bucaro, and C. M. Davis, Jr. Applied Optics Vol. 19 No. 22, 15 Nov. 1980, pps. 3746-3748. Yariv and Winsor had considered only the limiting case of an infinite nickel magnetostrictive jacket where it was assumed that the strains in the glass center are equal to the strains in the jacket material. The Jarzynski et al article presents calculations for the case of a finite thickness jacket for a number of magnetostrictive materials.
For some applications, particularly where large magnetic fields are present, high precision in the location of magnetic field lines is important. For example, in fusion reaction research, plasmas are confined in torus structures by powerful magnetic fields. It is important that the plasma be confined to particular volumes for maximum times while the plasma is heated. Misalignment of the magnets which generate the confining field will allow particles of the plasma to spiral away from the desired circular path around the circular axis of the torus, cool and impede experiments.
Heretofore, no method employing magnetostrictive interferometric devices to provide high precision mapping of magnetic field lines has been disclosed.